Method for producing an electronic module by sequential fixation of the components

ABSTRACT

The invention relates to a method for producing an electronic module ( 2 ) comprising a printed circuit ( 4 ) board ( 3 ), at least one first type of component ( 5 ), and a second type of component ( 6 ), said method comprising the following steps: solder is placed on the board; the first type of component is positioned; the solder is melted in order to solder the first type of component; the second type of component is positioned in such a way that it extends above the first type of component and has tongues ( 7 ) supported on the board by means of solder; and the solder is melted in order to solder the second type of component.

The present invention relates to a method for producing an electronicmodule that can be used for example in the motor vehicle field, and acorresponding production line.

BACKGROUND OF THE INVENTION

An electronic module usually comprises a printed circuit board ontowhich electronic components are soldered.

The current production methods include the steps of placing solder onthe board, usually in the form of a paste, positioning the components onthe latter and melting the solder by having the board run through areflow oven. This step of melting the solder is normally calledsoldering the components. The presence of components of large size onthe board or of concentrations of components in certain zones of theboard cause temperature gradients in the board, that is to say that thevarious zones of the board do not reach the set point temperature of thereflow oven at the same time. These temperature gradients cause heatstresses both on the board and on the components that have sustained thegreatest heating. The heat stresses may cause a degradation of theperformance and premature aging of the components. To prevent or limitthese temperature gradients, it is necessary to generate, according toeach type of module, specific heat profiles of the reflow oven that areawkward to control.

The largest components also limit the heat exchanges that ensure anoptimal melting of the solder by masking the smaller components or partsof the latter.

In addition, these larger components mask other components and prevent avisual inspection of these masked components.

Furthermore, with the known methods using a reflow oven, the run in thereflow oven limits the maximal height that the components may have.

In addition, the emergence of new standards relating to the compositionof the solder risk causing a rise in the solder melting temperature.This temperature rise risks increasing the heat stresses sustained bythe components. It will then be necessary to use, in order to producethe bodies of the components, technically enhanced plastics in order towithstand this temperature without damage. Certain components whoseperformance may be affected by an excess of heat will also have to beimproved.

OBJECT OF THE INVENTION

An object of the invention is to provide a means making it possible tolimit the heat stresses during the manufacture of an electronic module.

SUMMARY OF THE INVENTION

For this purpose, according to the invention, provision is made for amethod for producing an electronic module comprising a board with aprinted circuit, at least one component of a first type and a componentof a second type, the method comprising the steps of:

-   -   placing solder on the board,    -   positioning the component of the first type,    -   melting the solder in order to solder the component of the first        type,    -   positioning the component of the second type such that the        latter extends above the component of the first type and has        pads supported on the board by the solder,    -   melting the solder to solder the component of the second type.

Therefore, the component of the first type is soldered before thecomponent of the second type is put in place. The component of thesecond type therefore hampers neither the soldering of the component ofthe first type nor the operations of quality control of this soldering.For example therefore the bulky components will be positioned after thefirst soldering step. The component of the second type may protect thecomponent of the first type, particularly by forming a screen facing thecomponent of the second type. This is particularly valuable when thecomponent of the second type has a space requirement that is greaterthan the space requirement of the component of the first type. It isalso possible to prevent concentrations of components by spreading thesoldering of close components over the two soldering operations.

Advantageously, the component of the second type is soldered by applyingtwo electrodes to each pad of the component and by running an electricalcurrent between the electrodes to heat each pad of the component.

With this soldering method, only the pads of the component are heated,so that the component itself and the surrounding components sustainlittle or no temperature rise. It is therefore possible to usecomponents having relatively average temperature-resistance properties.

A further subject of the invention is a production line for applyingthis method. The production line comprises a unit for putting in placecomponents of the first type, a first heating unit for melting a solderplaced between the components of the first type and the circuit, a unitfor putting in place components of the second type and a second heatingunit for melting a solder placed between the components of the secondtype and the circuit.

Other features and advantages of the invention will emerge on readingthe following description of a particular, nonlimiting embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS Reference will be made to the appendeddrawings, amongst which:

FIG. 1 is a schematic view of a production line of electronic modulesaccording to the invention,

FIG. 2 is a partial schematic view in perspective of such a moduleduring the soldering of a component,

FIG. 3 is a view similar to that of FIG. 2 of this module after thesoldering of the component.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the figures, the production method according to theinvention is used on a production line, generally identified as 1, forproducing electronic modules generally identified as 2.

An electronic module 2 comprises a board 3 on which a conductive circuit4 has been formed in order to constitute, in a manner known per se, aprinted circuit board. The board 3 is of the IMS (insulated metalsubstrate) type. The circuit 4 may be formed of a conductive ink spreadby screen printing over the board 3. The circuit may also consist ofmetal tracks attached to the board 3.

The module 2 also comprises components of a first type 5 and a componentof a second type 6 comprising connection pads 7 of which only one isshown in FIGS. 2 and 3. The components 5, such as control components,are of smaller size than the component 6 which is for example a powercomponent. The component 6 in this instance extends above one of thecomponents 5. The connection pads, or more generally the connectionmembers, of the components 5 and 6 are attached to the circuit 4 bymelted solder 8. The solder 8 is in this instance solder paste made upof a mixture of tin and lead.

The production line 1 comprises a unit 10 for depositing the solder 8 onthe circuit 4 of the boards 3, a unit 20 for placing the components 5, aheating unit 30 consisting in this instance of a reflow oven, a unit 40for placing the components 6 and a unit 50 for soldering the components6.

The units 10, 20, 30 and 40 are known in themselves.

The soldering unit 50 comprises pairs of electrodes 51 connected to asource of electric power 52 in this instance delivering a current of theorder of 3000 amperes at a minimum frequency of 1000 hertz. Theelectrodes are in this instance made of cuprotungsten (25% copper and75% tungsten).

The various units are connected together in a conventional manner by aconveyor 60 transporting the boards 3.

The circuit 4 is already printed on the boards 3 when the latter reachthe production line 1.

In the unit 10, the solder 8 is deposited by screen printing on thelands for connecting the circuit 4 to the components 5 and 6.

The components 5 are then placed by the unit 20 on the circuit 4 so thatthe members for connecting the components 5 rest on the solder 8deposited on the corresponding connection lands of the circuit 3.

The board 3 thus fitted with the components 5 then passes into theheating unit 30 which melts the solder 8 and allows the soldering of thecomponents 5.

It is possible to carry out a visual inspection of the soldering of thecomponents 5 at the exit of the reflow oven 30. Since the component 6was not on the board during the run in the reflow oven 30, the weight tobe heated during this run is relatively small, which makes it possibleto use smaller capacity ovens or to run more boards through the ovensimultaneously.

The board 3 then travels into the unit 40 in which the component 6 ispositioned on the circuit 4 so that the pads 7 of the component 6 reston the solder 8 deposited on the corresponding connection lands of thecircuit 3.

The components 6 are then soldered by applying to each pad 7 of thecomponents 6 a pair of electrodes 51 in order to run an electricalcurrent between said electrodes via the corresponding pad 7 and heateach pad 7 of the component 6 sufficiently to cause the solder 8 tomelt. It will be noted that the pressure of the electrodes on the partto be soldered ensures that the part to be soldered is properly pressedonto the circuit. As an example, the electrodes may exert a force of 12daN on the part to be soldered.

Melting the solder by means of the electrodes has the advantage of beingextremely fast (less than a second) while a run through the reflow ovenlasts approximately one minute.

It is possible to influence the quality of the soldering or to adapt thesoldering to the materials or the components to be soldered by modifyingthe soldering profile (and particularly the current intensity curveaccording to the time, the mechanical pressure of the electrodes on thecomponent pads), the geometry of the electrodes (in order in particularto allow a better dispersal of the calories), the force exerted on thepart (in order to maintain contact between the electrode and the partand prevent the formation of a spark), the spacing and positioning ofthe electrodes on the pads (particularly in order to modify the routetaken by the current in the pads). The temperature of the electrodes iskept substantially constant and relatively low, of the order of 40° C.,in order to maintain the same soldering conditions for all the parts,hence the value of providing means for making it easier to disperse thecalories at the electrodes, such as an appropriate geometry.

Naturally, the invention is not limited to the embodiment described andit is possible to apply variant embodiments thereto without departingfrom the context of the invention as defined by the claims.

In particular, it is possible to deposit the solder used for solderingthe components 6 after soldering the components 5 by a screen printingoperation. The solder may also be deposited directly on the pads of thecomponents before they are put in place on the circuit.

It is possible to heat the paste of the components in a differentmanner, for example by applying a soldering iron to each pad of thecomponents 6 or of the components 5. The heating unit 50 may be a reflowoven.

The electrodes may be made from different metals, particularly fromcopper, tungsten, molybdenum, etc. The electrodes may therefore be madeof cuprotungsten (25% copper and 75% tungsten) as in the embodimentdescribed or be made of copper and comprise a tungsten tip for servingas the point of contact with the part to be soldered.

The parts to be soldered may be made of copper, brass, an alloy or ametal that may or may not be tinned, etc. The board may be of the INStype as described or be another type and for example comprise a glassand epoxy resin screen circuit like the boards of the FR4 type.

It is possible to use other solders, tin-based, silver-based, leadlessetc. Solder may also be deposited in the form of a metal strip either onthe circuit or on the members for connection of the components.

The modules may of course have a structure different from that of theabove embodiment which has been specified only to explain the inventionand is absolutely not limiting. The components 6 placed after the boardshave run through the reflow oven 30 may be bulky components orcomponents that could be damaged by the heat prevailing in the oven. Thecomponents 6 may be identical to the components 5 but solderedsubsequently in order to prevent too great concentrations of componentsduring the first soldering.

1. A method for producing an electronic module comprising a board with aprinted circuit, at least one component of a first type and a componentof a second type: placing solder on the board, positioning the componentof the first type, melting the solder in order to solder the componentof the first type, positioning the component of the second type suchthat the latter extends above the component of the first type and haspads supported on the board by the solder, melting the solder to solderthe component of the second type.
 2. The method as claimed in claim 1,wherein the component of the first type is soldered by running the boardin a reflow oven.
 3. The method as claimed in claim 1, wherein thecomponent of the second type is soldered by applying two electrodes toeach pad of the component of the second type and by running anelectrical current between the electrodes to heat each pad of thecomponent.
 4. The method as claimed in claim 3, wherein a phase ofadapting the solder to the component of the second type is carried outby modifying at least one of the parameters selected from a groupconsisting of a solder profile, a current intensity curve of theelectrodes, a mechanical pressure of the electrodes on the pads of thecomponent of the second type, a geometry of the electrodes, and aspacing and positioning of the electrodes on the pads of the component.5. The method as claimed in claim 1, wherein the solder used forsoldering the component of the second type is placed on the circuit ofthe board at the same time as the solder used for soldering thecomponent of the first type.
 6. The method as claimed in claim 5,wherein the solder is a solder paste.
 7. The method as claimed in claim1, wherein the solder is placed on the pads of the component of thesecond type before the component of the second type is positioned. 8.The method as claimed in claim 1, wherein the component of the secondtype extends above the component of the first type.